The human body contains many joints that permit articulation of varying degrees between bones. Those that permit free articulation are referred to as diarthroses. Examples include the hip, knee, elbow and shoulder. A variety of connective tissues are associated with the diarthroses joints, including intra-articular cartilages that provide cushioning and smooth sliding surfaces, ligaments that provide flexible connections between bones, and tendons that slide over joints and connect the muscles to provide motion. When connective tissues are compromised, joint pain and loss of function can result.
Osteoarthritis, or OA, is one of the most common causes of disability in the United States. OA is sometimes referred to as degenerative, or wear and tear, arthritis. OA is characterized by the breakdown of the articular cartilage within the joint. Over time, the cartilage may wear away entirely, resulting in bone-on-bone contact. Since bones, unlike cartilage, have many nerve cells, direct bone contact can be very painful to the OA sufferer. In addition to the pain and swelling, the OA sufferer can experience a progressive loss of mobility at the joint. This is due to loss of the joint space, where the articular cartilage has completely worn away.
Currently, various medications are often recommended to reduce the swelling and pain of OA. Other treatments such as weight loss, braces, orthotics, steroid injections and physical therapy may also help alleviate pain and restore function by strengthening muscles supporting the joint. However, since articular cartilage is avascular, or lacks a blood supply, repair and growth of adult cartilage is minimal. If the pain or immobility becomes too severe and other therapies do not alleviate the symptoms, surgical interventions become necessary. Surgical treatments include arthroscopy to clean the joint by removing loose fragments, osteotomy, or joint replacement.
Shoulder Anatomy
There are two joints in the shoulder. The gleno-humeral joint (also called the shoulder joint) is a ball-and-socket joint formed by the scapula and the head of the humerus bone (FIGS. 1A-E). This joint allows the shoulder to move forward and backward and the arm to move in a circular motion. The “socket” (the glenoid fossa of the scapula) is shallow, covering only a third of the “ball” (the head of the humerus). The socket is deepened by the glenoid labrum (see, e.g., FIG. 1C). The labrum is a fibro-cartilaginous rubbery structure that encircles the glenoid cavity effectively deepening the socket to increase static stability of the gleno-humeral joint. The acromioclavicular joint is formed by a part of the scapula called the acromion and the clavicle (see, e.g., FIG. 1A).
The acromioclavicular joint capsule is a soft tissue envelope that encircles the gleno-humeral joint and attaches to the scapula, humerus, and head of the biceps. It is lined by a thin, smooth synovial membrane.
The rotator cuff is a group of four tendons that connects muscles from the scapula and allows the shoulder to rotate and elevate. The four rotator cuff muscles—the subscapularis, supraspinatus, infraspinatus and teres minor muscles—provide support for the gleno-humeral joint.
The deltoid muscle (see, e.g., FIGS. 1B-C) is the largest, strongest muscle of the shoulder. It originates in three portions, the anterior, middle and posterior portions. The anterior portion flexes and medially rotates the arm, the middle portion abducts the arm and the posterior portion extends and laterally rotates the arm. The deltoid muscle takes over lifting the arm once the arm is away from the side of the body.
Rotator cuff tears, the most common injury of the shoulder, cause morphologic changes to cuff tendons and muscles, which can alter muscle architecture and moment arm. These alterations can affect shoulder performance in terms of muscle force and joint strength. Rotator cuff tears are often accompanied by tears in the glenoid labrum due to the alterations in the biomechanics of the shoulder joint.